Motor Capacitor Sizing Calculator

About the Motor Capacitor Sizing Calculator

This motor capacitor sizing calculator estimates start and run capacitor microfarad (μF) values for single-phase motor replacement. Use it when replacing a failed capacitor on a single-phase motor (HVAC, pool pumps, well pumps, compressors) where the original capacitor is missing or unreadable.

For exact replacement, always use the manufacturer’s specified capacitor rating. Use this calculator as a starting estimate when the original spec is unknown — then verify with a multimeter capacitance reading on the original capacitor if possible.

How capacitor sizing works

Start capacitor: ~150–200 μF per HP
Run capacitor: ~25–40 μF per HP

Start capacitors deliver high-instantaneous reactive current to break the motor away from rest — they’re typically 100–400 μF and rated for short-duration use (a few seconds per start). Run capacitors stay in the circuit continuously, providing balanced two-phase operation; they’re smaller (5–80 μF) and rated for continuous duty. Voltage rating is critical: must equal or exceed the motor’s running voltage.

Worked example

A 1 HP, 230V single-phase pool pump motor with a failed start capacitor: estimated start cap = 1 × 200 = ~200 μF, rated ≥ 250 VAC. Verify against the motor manufacturer’s spec or a similar known-good motor before installing.

When to use this calculator

• Replacing a failed start or run capacitor on a single-phase motor
• Estimating capacitor μF when the nameplate is unreadable
• Troubleshooting motors that won’t start (often a failed start cap)
• Specifying replacement capacitors for HVAC condenser fan motors
• Sourcing capacitors for legacy or obsolete motor models
• Quick reference during HVAC and refrigeration service calls

Capacitor types and applications

• Start capacitor (electrolytic): high μF (100–400), short-duty, used during start only. Round metal case, oil-filled.
• Run capacitor (oil-filled or metallized polypropylene): smaller μF (5–80), continuous duty, stays in circuit. Rectangular metal case typically.
• Dual-run capacitor: two run caps in one package (e.g., 35+5 μF), common in HVAC condensers for compressor + fan motor.
• Hard-start kit: adds a start capacitor + relay to a PSC (run-only) motor for difficult start conditions like AC condenser short-cycling.

Frequently Asked Questions

What’s the difference between a start and run capacitor?

Start capacitors deliver high reactive current briefly during motor startup, then drop out via centrifugal switch or relay. Run capacitors stay in the circuit continuously, providing balanced two-phase operation. Start caps are larger μF; run caps are smaller but rated for continuous duty.

How do I size a replacement capacitor?

Match the original spec exactly if possible (microfarad value ±5%, voltage ≥ original). If unknown, use 150–200 μF per HP for start, 25–40 μF per HP for run as a starting estimate. Always match or exceed the voltage rating.

Can I use a higher voltage capacitor than spec?

Yes — higher voltage rating is fine and often improves longevity. A 370 VAC cap can replace a 250 VAC cap. NEVER go lower — that risks failure or fire.

Can I use a slightly different microfarad value?

For run capacitors, stay within ±5–10%. Larger μF gives more torque but more heat; smaller μF reduces both. For start capacitors, ±20% is usually acceptable; the duration is short enough that exact matching matters less.

How do I know if a capacitor is bad?

Visual: bulging, leaking, or burnt smell. Multimeter (capacitance setting): reading should be within 6% of nameplate μF. If the motor hums but won’t start, the start cap is the most likely culprit.

What does “uF” or “MFD” mean?

Microfarad — the unit of capacitance. μF, uF, and MFD all mean the same thing on capacitor labels. 1 microfarad = 0.000001 farads.

Related calculators and Electracore products

Replacing a single-phase motor entirely? Browse General Purpose or Pump motors from Baldor, Marathon, and other brands.

Related calculators: HP to Amps Calculator · Motor FLA Calculator · Motor Torque Calculator · Motor Efficiency Calculator.